Coded reverse link messages for closed-loop power control of forward link control messages

ABSTRACT

Multiple field units in a CDMA system are synchronized for communication with a base station using shared forward and reverse link channels. In an illustrative embodiment, each field unit is assigned a time slot in a forward link channel to receive messages from the base station. Likewise, each field unit is assigned a time slot on a common reverse link channel for transmitting messages to the base station. Timing alignment and power level control among each of many field units and the base station is achieved by analyzing coded transmissions received at the base station in a corresponding time slot as transmitted by each field unit. The codes may be orthogonal, pseudonoise (PN), or other codes. The power level of forward control channel messages can thus be individually controlled. In this way, minimal resources are deployed to maintain communication and precise synchronization between a base station and each of multiple users, minimizing collisions between field units transmitting in adjacent time slots on the reverse link.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. application Ser. No.10/137,116 filed on May 1, 2002 which in turn claims the benefit of U.S.Provisional Application No. 60/288,927 filed on May 4, 2001 which areincorporated by reference as if fully set forth.

BACKGROUND

Increasing use of wireless telephones and personal computers has led toa corresponding increase in demand for advanced telecommunicationservices that were once thought practical only for specializedapplications. In the 1980s, wireless voice communication became widelyavailable through cellular telephone networks. Such services werethought at first to be for the exclusive province of businessmen becauseof expected high subscriber costs. The same was also true for access toremotely distributed computer networks, whereby until very recently,only business people and large institutions could afford the necessarycomputers and wireline access equipment.

As a result of the widespread availability of affordable newtechnologies, the general population now increasingly desires to havenot only wireline access to networks such as the Internet and privateintranets, but also wireless access as well. Wireless technology isparticularly useful to users of portable computers, laptop computers,hand-held personal digital assistants and the like who prefer access tosuch networks without being tethered to a telephone line.

There still is no widely available satisfactory solution for providinglow cost, high speed access to the Internet, private intranets, andother networks using the existing wireless infrastructure. This is mostlikely an artifact of several unfortunate circumstances. First, thetypical manner of providing high speed data service in the businessenvironment over a wireline network is not readily adaptable to thevoice grade service available in most homes or offices. For example,such standard high speed data services do not necessarily lendthemselves to efficient transmission over standard cellular wirelesshandsets because wireless networks were originally designed only toprovide voice services. As a result, present day digital wirelesscommunication systems are optimized for voice transmissions, althoughcertain schemes such as CDMA do provide some measure of asymmetricalbehavior for the accommodation of data transmissions. For example, thedata rate specified by the Telecommunication Industry Association (TIA)for IS-95 on the forward traffic channel is adjustable in incrementsfrom 1.2 kbps up to 9.6 kbps for so-called Rate Set 1, and incrementsfrom 1.8 kbps up to 14.4 kbps for Rate Set 2. On the reverse linktraffic channel, however, the data rate is fixed at 4.8 kbps.

At best, existing wireless systems therefore typically provide a radiochannel that can accommodate maximum data rate transfers of 14.4kilobits per second (kbps) over a forward link direction. Such a lowdata rate channel does not lend itself directly to transmitting data atrates of 28.8 or even 56.6 kbps that are now commonly available usinginexpensive wireline modems, not to mention even higher rates such asthe 128 kbps that are available with Integrated Services Digital Network(ISDN) type equipment. Data rates at these levels are rapidly becomingthe minimum acceptable rates for activities such as browsing web pages.

Although wireline networks were known at the time when cellular systemswere initially developed, for the most part, there was no provision madefor such wireless systems to provide higher speed ISDN- or xDSL-gradedata services over cellular network topologies.

In most wireless systems, there are many more potential users than radiochannel resources. Some type of demand-based multiple access system istherefore required.

Whether the multiple access is provided by the traditional FrequencyDivision Multiple Access (FDMA) using analog modulation on a group ofradio frequency carrier signals, or by schemes that permit sharing of aradio carrier frequency using Time Division Multiple Access (TDMA), orCode Division Multiple Access (CDMA), the nature of the radio spectrumis such that it is expected to be shared. This is quite dissimilar tothe traditional environment supporting data transmissions in which thewireline medium is relatively inexpensive and is not typically intendedto be shared.

Other factors to consider in the design of a wireless system are thecharacteristics of the data itself. For example, consider that access toweb pages generally is burst-oriented, with asymmetrical data ratetransmission requirements in a reverse and forward direction. In acommon application, a user of a remote client computer first specifiesthe address of a web page to a browser program. The browser program thensends the web page address data, which is usually 100 bytes or less inlength, over the network to a server computer. The server computer thenresponds with the content of the requested web page, which may includeanywhere from 10 kilobytes to several megabytes of text, image, audio,or even video data. The user thereafter may spend several seconds oreven several minutes reading the content of the page before downloadinganother web page.

In an office environment, the nature of most employees' computer workhabits is typically to check a few web pages and then to do somethingelse for an extended period of time, such as accessing locally storeddata or even terminating use of the computer altogether. Therefore, eventhough such users may remain connected to the Internet or privateintranet continuously during an entire day, actual use of the high speeddata link is usually quite sporadic.

If wireless data transfer services supporting Internet connectivity areto coexist with wireless voice communication, it is becomingincreasingly important to optimize the use of available resources inwireless CDMA systems. Frequency re-use and dynamic traffic channelallocation address some aspects of increasing the efficiency of highperformance wireless CDMA communication systems, but there is still aneed for more efficient utilization of available resources.

SUMMARY

A significant limitation of forward link capacity involves the amount ofcarrier power that can be allocated to dedicated traffic payloadchannels. Overhead channels such as pilot and paging consume power thatmay be otherwise utilized for transmitting data to users. A majorlimitation of so-called common channels in CDMA systems is the lack ofpower control associated with the messages to the individual users.Power control allows an increase in capacity as only the power on aper-user basis is allocated, allowing the residual power to be used fortraffic payload. Common channels such as the paging channel have nomethod of closed-loop power control feedback to the base station.Because of this, enough power must be allocated to all messages to meetthe minimum performance for all users in the network. This causessignificant waste as much of this power may be reallocated to traffic.

Users can require on-demand and sporadic high speed throughput of dataon a wireless communication link. For example, remote users can beconnected to the Internet over a wireless link that supports on-demandhigh speed throughput capability for downloading an object file such asa web page. Such users can remain in a standby mode when no datapayloads are transmitted in a reverse link direction. To support suchusers, it is advantageous to maintain synchronization with a basestation even while the link is not actively being used to transmit orreceive data. This can be achieved by maintaining a minimal connectionwith the base station even when no data is being actively transferredbetween the base station and a specific field unit. A shared channel inthe reverse link called a heartbeat channel can be used to maintain theminimal connection by transmitting a minimal indication from a fieldunit to keep it synchronized with the base station.

This invention is a method of providing closed-loop feedback formessages on the forward link meant to control users in a standby modeutilizing a reverse link time slotted heartbeat channel. In theheartbeat channel, during the user's time slot, one of several codes maybe transmitted. One code (“heartbeat”) is used to notify the basestation that the field unit desires to remain in a standby mode. Anothercode (“heartbeat with request to go active”) is used to notify the basestation that the field unit is ready to begin transmitting a datapayload to the base station. Since the duration of these time slots isadequate to support more than one code, additional signaling may beprovided by utilizing additional codes. For instance, codes to indicatea power up/power down power control scheme may be sent simultaneouslywith the heartbeat and heartbeat with request messages. This is providedby arranging the messages in a code matrix where one axis indicatesheartbeat or heartbeat with request, and the other axis indicates powerup/power down to the base station.

Accordingly, a method for supporting wireless communications includesallocating a common control channel to support synchronizedcommunications from a transmitter to multiple receivers and assigning atime segment in which the transmitter communicates an indication to atarget receiver by generating a signal at an adjusted power level overthe common control channel for each of the respective receivers.Information indicating whether to increase or decrease power leveltransmissions for the control channel communications at the transmitteris transmitted as an encoded signal for each receiver.

The invention allows feedback from a field unit to the base station toprovide closed loop power control of the individual transmitted messageson a per-user basis. Additionally, other information may be conveyed inthis manner.

For example, a coding matrix may communicate a request to remain inheartbeat or to make a request to go active. In addition, the field unitmay communicate power control information indicating whether powerslevels for a forward link control message should be increased or shouldbe decreased in power. These four conditions can therefore be handled byencoding the transmission with four different message codes.

The message codes may be orthogonal codes, such as Walsh codes, othertypes of codes such as quasi-orthogonal pseudonoise (PN) codes, or maybe PN codes.

The point is, both the desired heartbeat state, as well as the powercontrol feedback messages for the control channel, can be handled byencoding the transmissions in the appropriately assigned time slot of areverse link control channel, such as heartbeat standby or heartbeatrequest active channel.

This scheme provides a method for providing closed loop feedback so thatpower control on even a commonly shared forward control channel may beimplemented. Therefore, one major limitation of managing forward linkcapacity is eliminated, since only enough power needs to be allocated toindividual messages to meet the minimum performance for individual usersin the system, rather than the minimum performance for all userscollectively.

According to another aspect of the invention, feedback from a field unitto the base station provides closed loop power control of a dedicatedcontrol channel on a forward link using encoded transmissions on ashared control channel of a reverse link.

BRIEF DESCRIPTION OF THE DRAWING(S)

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 is a general diagram illustrating a wireless communication systemaccording to the principles of the present invention.

FIG. 2A is a timing diagram illustrating heartbeat and Link QualityManagement (LQM) slot timing according to the principles of the presentinvention.

FIG. 2B is a table illustrating an assignment of codes to heartbeat andheartbeat request active channel messages.

FIG. 3 is a diagram illustrating an exemplary bit definition of an LQMslot according to the principles of the present invention.

FIG. 4 is a graph illustrating a field unit requesting to go active andthe allocation of traffic channels to transmit a data payload in areverse link direction according to the principles of the presentinvention.

FIG. 5 is a block diagram supporting channel synchronization accordingto the principles of the present invention.

FIGS. 6A and 6B are flow charts illustrating how forward and reversechannels are synchronized according to the principles of the presentinvention.

FIG. 7 is a graph illustrating pulse sampling techniques for identifyinga timing mark for synchronizing forward and reverse channels accordingto the principles of the present invention.

FIG. 8 is a table showing attributes of an active, standby and idle modefor synchronizing a field unit to a base station according to theprinciples of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 1 is a diagram of a wireless communication system 100 according tothe principles of the present invention. A base station 25 maintainswireless communication links with each of a plurality of field units42A, 42B, 42C (collectively, field units 42) as shown. Such wirelesslinks are established based upon assignment of resources on a forwardlink 70 and a reverse link 65 between the base station 25 and fieldunits 42. Each link 65 or 70 is typically made up of several logicalreverse link channels 55 and several logical forward link channels 60.

As shown, communication system 100 supports wireless communicationbetween an interface 50 and network 20. Typically, network 20 is aPublic Switched Telephone Network (PSTN) or computer network such as theInternet, internet or intranet. Interface 50 is preferably coupled to adigital processing device such as a portable computer 12, to providewireless access to the network 20. Consequently, portable computerdevice 12 has access to network 20 based on communications over acombination of both hard-wired and wireless data links.

In a preferred embodiment, the forward link channels 60 and reverse linkchannels 55 are defined in wireless communication system 100 as CodeDivision Multiple Access (CDMA) channels. That is, each CDMA channel ispreferably defined by encoding and transmitting data over the channelwith an augmented pseudo random noise (PN) code sequence. The PN codeddata is then modulated onto a radio frequency carrier. This enables areceiver to decipher one CDMA channel from another knowing only theparticular augmented PN code assigned for a given channel. In accordancewith the preferred embodiment, each channel preferably occupies a 1.25MHz band consistent with the IS-95 CDMA standard and is capable oftransmitting at 38.4 kbps.

Forward link channels 70 include at least three logical channels. Asshown, this includes a forward control message channel, such as a LinkQuality Management (LQM) channel 60L, a paging channel 60P, and multipletraffic channels 60T.

Reverse link 65 includes a heartbeat standby channel 55HS, heartbeatrequest active channel 55HRA, access channel 55A and multiple trafficchannels 55T. Generally, the reverse link channels 55 are similar to theforward link channels 60 except that each reverse link traffic channel55T can support variable data rates from 2.4 kbps to a maximum of 160kbps.

Data transmitted between base station 25 and field unit 42A typicallyconsists of encoded digital information, such as hypertext transferprotocol (HTTP) encoded Web page data. Based on the allocation ofmultiple traffic channels in the reverse link 65 or forward link 70,higher data transfer rates can be achieved in a particular link betweenthe base station 25 and field unit 42A. However, since multiple fieldunits 42 compete for bandwidth allocation, a field unit 42A may have towait until resources are free to be assigned traffic channels totransmit a data payload.

As shown in FIG. 2A, the forward link LQM channel 60L is partitionedinto a predetermined number of periodically repeating time slots 310 forthe transmission of messages to each of multiple field units 42. Eachfield unit 42 identifies messages directed to itself based upon messagesreceived in its assigned time slot 310. In other words, field units 42monitor messages received in their respectively assigned time slots 310to receive information from base station 25.

The reverse link heartbeat standby channel 55HS and heartbeat requestactive channel 55HRA are also shared among multiple users. Thesechannels are both partitioned into periodically repeating time slots 310so that the time slots 310 in each heartbeat channel align with eachother and also the time slots of the LQM channel 60L. A time slot 310 ofthe reverse link heartbeat channels 55HS or 55HRA is assigned to one ofmany field units 42 for transmitting heartbeat-type messages to the basestation 25 over either the heartbeat standby channel 55HS or heartbeatrequest active channel 55HRA. Accordingly, the base station 25identifies from which field unit 42A a message is transmitted based uponthe receipt of a message in a particular time slot.

The pair of shared channels in the reverse link are called heartbeatchannels because one aspect of the present invention involvestransmitting a minimal indication from a field unit 42A to keep itsynchronized with the base station 25. The heartbeat channels 55HS,55HRA and LQM channel 60L are described in more detail below.

As previously mentioned, another aspect of the present inventioninvolves maintaining a minimal maintenance link between each of multiplefield units and a base station 25 even when they are not presentlytransmitting a data payload in a reverse link direction. This schememaintaining synchronization is particularly advantageous in applicationswhere each of the multiple field units sporadically request to go activeand transmit data in a reverse link. Since each of the field units 42 isalready synchronized with the base station 25 via the minimal link, afield unit 42A can be assigned reverse link traffic channels 55 and,when assigned, almost immediately transmit a data payload in a reverselink direction without interfering with other channels. That is, a fieldunit 42A does not need to go through a lengthy process ofre-synchronizing itself with the base station 25 when traffic channelsare eventually assigned for its use.

In the following description, reference is again generally made to FIG.1, but more specific details of LQM channel 60 and heartbeat channel 55Hare referenced to FIG. 2A.

To establish a synchronized link with base station 25, field units 42transmit messages on access channel 55A to base station receiver 35 viafield unit transmitter 40. These messages are then acknowledged andprocessed at base station 25. If available, resources are allocated bybase station 25 to establish a bi-directional communication link withthe requesting field unit 42A.

Within the forward link 70, the paging channel 60P is used by the basestation transmitter 30 to send overhead and paging messages or commandsto the field unit receiver 45. Overhead information includes data suchas system configuration parameters for establishing wireless links withfield units 42.

As mentioned previously, wireless communication system 100 includes aheartbeat channel 55HS and heartbeat request active channel 55HRA in thereverse link 65 and link quality management channel (LQM) 60L in theforward link 70. These channels are shared between the base station 25and multiple field units 42. That is, the base station 25 transmitsmessages to multiple field units 42 using the same forward link LQMchannel 60L, where a message to a particular field unit 42A istransmitted in an assigned time slot 310. In this way, time slotassignments serve as a way of addressing messages to a particular fieldunit and corresponding communication link.

The principles of the present invention are advantageously deployed tosupport users that require on-demand and sporadic high speed throughputof data on a wireless communication link. For example, remote users atPC device 12 can be connected to the Internet over a wireless link thatsupports on demand high speed throughput capability for downloading anobject file such as a web page. Users can remain in a standby mode whenno data payloads are transmitted in a reverse link direction. Forexample, use of a link can be minimal for a period of time so that theuser can review a web page. To support such users, it is advantageous tomaintain synchronization with the base station 25 even while the link isnot actively being used to transmit or receive data. This is achieved inwireless communication system 100 by maintaining a minimal connectionwith the base station 25 even when no data is being actively transferredbetween the base station 25 and a specific field unit 42A.

One aspect of the minimal connection between a field unit 42A and basestation 25 involves adjusting timing of the field unit 42A so that itstiming is properly aligned with base station 25. Another aspect of theminimal connection involves adjusting the power level output of thefield unit 42A so that it transmits at a low but detectable power level.

As mentioned, repeatedly creating or reviving connections for users whosporadically need a link can be time consuming and result in theinefficient use of resources. It is also inefficient to reserveresources such as traffic channels 55T on a continuous basis forsubscribers who are not presently transmitting data. Accordingly,traffic channels 55T are allocated on an as-needed basis to support datatransfers, optimizing the use of available resources in wirelesscommunication system 100.

FIG. 2A is a timing diagram more particularly illustrating the heartbeatstandby channel 55HS, heartbeat request active channel 55HRA and LQMchannel 60L. Preferably, there are two LQM channels 60L combined with atotal of four heartbeat-type channels including two heartbeat standbychannels 55HS and two heartbeat request active channels 55HRA sincecoded channels are typically allocated in pairs. However, only one ofeach channel type is shown in FIG. 2A for illustrative purposes. Ofcourse, the paired sets of channels can be used to support twice thenumber of users.

As shown, 64 time slots (in each direction) are defined per EPOCH periodin each of the heartbeat standby 55HS, heartbeat request active channel55HRA and LQM 60L channels. Up to 48 field units 42 in the standby modecan be supported along with up to 16 users in the active mode. The EPOCHperiod in the illustrated embodiment is 13.3 mS, so that each time slotis 208 mS or 256 PN code chips. Because time slots repeat on a periodicbasis, the base station 25 can exchange information with a particularfield unit 42 every EPOCH or 13.3 mS.

Data transmissions on the LQM channel 60L are maintained by base station25, which is preferably used as a master timing reference. Field units42, therefore, must synchronize themselves to base station 25, andspecifically to the LQM channel 60L, in order to communicate with thebase station 25 and transmit within an assigned time slot.

Generally, a link between base station 25 and a field unit 42A ismaintained in one of three modes: active, standby or idle. Precisesynchronization between the base station 25 and a particular field unit42A is maintained only for field units 42 in the active and standbymode. FIG. 7 provides more details about mode types maintained for aparticular link between the base station 25 and a field unit 42A. Thisaspect of the present invention will be discussed later in thespecification.

Each field unit 42A in the standby or active mode is assigned one timeslot in the forward link LQM channel 60L and one time segment in thereverse link heartbeat-type channels. Accordingly, information istargeted to a field unit 42A based upon the transmission of a message ina particular time slot. For example, a field unit 42A assigned to timeslot #1 demodulates and decodes information received in time slot #1 onthe forward link LQM channel 60L, while data transmitted back to basestation 25 is transmitted by field unit 42A in time slot #1 of thereverse link heartbeat standby channel 55HS or heartbeat request activechannel 55HRA. Both base station 25 and field unit 42A identify to whichlink a message is directed based on receipt of a message in a particulartime slot 310.

Preferably, there is a timing offset between time slots in eachrespective channel, allowing the base station 25 time to process amessage received in an assigned time slot and then respond accordinglyover the LQM channel 60L in a following portion of a cycle. Thus,messages transmitted over the LQM channel 60L include feedback messagesthat are used to adjust transmitting characteristics of a field unit42A.

It should be noted that although the LQM channel 60L is used as a timingreference as described above, the principles of the present inventionequally apply where the heartbeat-type channels 55HS and 55HRA are usedin a forward link and LQM-type channel is used in a reverse link. Inother words, base station 25 is optionally synchronized with respect toa field unit 42A.

In the standby mode, synchronization is maintained between the forwardlink LQM channel 60L and reverse link heartbeat standby channel 55HSbased upon messages sent in the appropriate time slot on the LQM channel60L indicating to a particular field unit 42 whether messagestransmitted to the base station 25 from that field unit 42 are receivedin the appropriate time slot. For example, message transmissions fromthe field unit transmitter 40 to base station 25 are analyzed at basestation receiver 35 to achieve fine tuning alignment between the basestation 25 and each of multiple field units 42.

As shown in FIG. 2A, time slots A₁ through A₁₆ of the LQM channel 60Lare reserved for field units 42 in the active mode, indicating thattraffic channels are assigned to a field unit 42A in a reverse linkdirection and data is being transferred from the field unit 42 to thebase station 25. Contrariwise, time slots 1-48 of the LQM channels 60Lare reserved for field units 42 operating in the standby mode that arenot presently transmitting a data payload over a reverse link ofcommunication system 100.

At any given time, there are preferably no more than 48 of the 64 timeslots of the heartbeat channel 55H or LQM channel 60L assigned torespective field units 42. This ensures that on completion of a datatransfer between a field unit 42A and base station 25, a field unit 42Ain the active mode assigned an active time slot can revert back to thestandby mode and consequently be assigned an unused standby mode timeslot 310 again.

Preferably, field units 42 in the standby mode are assigned an unusedactive time slot 310 as close to the EPOCH mark M1 as possible when theyare placed in the active mode. For example, if 48 field units areassigned standby mode LQM slots S₁, S₂, . . . S₄₈, a field unit 42A setto the active mode would be assigned active mode time slot A₁ in the LQMchannel. The next active time slot 310 to be assigned to a field unit42A would be the lowest numbered and unused time slot such as A₂,assuming A₁ is then in use.

It should be noted that heartbeat standby channel 55HS also includesadditional time slots for transmitting messages from an active fieldunit 42A, i.e., a field unit 42A transmitting data in a reverse linkover assigned traffic channels, to base station 20. Preferably, reverseLQM time slots 250 are allocated for transmitting link qualityinformation from a corresponding active field unit 42A to base station20. In this way, base station 20 can be notified of a corresponding linkquality of transmissions on forward channels between the base station 20and field unit 42.

In a specific application utilizing the reverse LQM time slots 250, afield unit 42A can monitor the quality of a forward link signal from thebase station 20 and transmit a modulated message including forward errorcorrection information to the base station 20 in an assigned LQM timeslot 250. Based on these feedback messages transmitted in an LQM timeslot 250, properties of the transmitted signal from base station 20 canbe adjusted so that subsequent messages on the forward link channels tothe field unit 42A can be properly detected. For instance, field unit42A can monitor whether a signal transmitted by the base station 20 on aforward link traffic channel is transmitted at an appropriate powerlevel so that the power level of the received signal is within a desiredrange, e.g., a selected signal-to-noise ratio. In this instance, themessage sent in the reverse LQM time slot 250 can indicate whether basestation 20 should increase or decrease its power level output on theforward channel.

The heartbeat standby channel 55HS therefore supports at least two typesof communications between multiple field units 42 and base station 20. Afirst field unit 42A in the standby mode transmits a timing referencesignal that is monitored at base station 20 for adjusting timingalignment of the corresponding filed unit 42A. As recently discussed, asecond field unit 42B in the active mode is assigned a reverse LQM timeslot 250 for transmitting a message to base station 20. Preferably, themessage transmitted in a reverse LQM time slot 250 includes a datamessage that is demodulated and decoded at base station 25 to determinethe contents of the message.

The mere RF (Radio Frequency) transmission in a time slot on theheartbeat standby channel 55HS by a field unit 42A in the standby modeitself is an indication to the base station 20 that the field unit 42Adesires to remain in the standby mode. As mentioned, the lattertransmission by a field unit 42A in the standby mode preferably does notinclude an encoded and modulated message including forward errorcorrection information.

FIG. 3 is a timing diagram illustrating an exemplary mapping of bits ina forward link LQM time slot 310 according to the principles of thepresent invention. As shown, there are 16 bits transmitted in each timeslot 310, although this can vary depending on the application. One bitof the LQM time slot 310 is LQM timing bit 311 that indicates whether afield unit message transmission received at the base station 25 on alast message cycle is accurately received within an assigned time slot310. This ensures that other field units 42 transmitting messages inadjacent time slots of the same reverse link channel 65 do not interferewith each other.

In a preferred embodiment, the LQM timing bit 311 indicates whether afield unit 42A is to advance or retard its timing on the reverse link65. A logic one indicates that timing should be advanced ⅛ of a chipwhile a logic zero indicates that timing should be retarded ⅛ of a chip.In this way, the base station 25 individually synchronizes communicationlinks between the base station 25 and each of a plurality of field units42. Said differently, timing of message transmissions from correspondingfield units 42 are frequently adjusted, so that corresponding messagesare received in the assigned time slots at the base station 25.Consequently, a field unit 42 can synchronize itself with the basestation 25 even though it is moving very fast relative to the basestation 25.

In a preferred embodiment, the base station 25 transmits information onthe LQM channel 60L based on BCH coding. This enables a receiving fieldunit 42 to detect and correct errors. For example, the use of a 15,7code allows up to 2 errors to be corrected and up to 3 errors to bedetected. As shown in FIG. 3, there are 8 parity bits 313 for errorcorrection and detection.

Referring again to FIG. 2A, a timing diagram illustrates the heartbeatstandby channel 55HS and heartbeat request active channel 55HRA. Asshown, time slot numbering is selected for both channels so that theyline up with each other. For example, time slot #1 for each heartbeatchannel are aligned with each other in a given time segment, T_(SLOT).

The heartbeat standby channel 55HS and heartbeat request active channel55HRA serve different functions. For example, a field unit 42A assigneduse of a particular time slot 310 transmits over the heartbeat standbychannel 55HS in order to provide an indication to base station 25 thatthe field unit 42 desires to remain in the standby mode. On the otherhand, a field unit 42A alternatively transmits over the appropriate timeslot 310 of the heartbeat request active channel 55HRA to provide anindication to the base station 25 that the field unit 12 desires theallocation of reverse link traffic channels for transmitting a datapayload from the field unit 42A to base station 25.

FIG. 2B is a table illustrating in more detail how the communication onthe heartbeat standby channel 55HS and heartbeat request active channel55HRA may be encoded to provide different indications to the basestation 25 of certain conditions in the field unit 12. For example, acoding matrix as shown may be used to communicate four differentmessages. One axis of the matrix represents communicating a request toremain in heartbeat mode or a request to go into an active mode. Theother axis indicates power control information for forward link controlmessages, such as whether control messages transmitted on a sharedforward link control channel, such as the LQM channel 60L, should beincreased in power or decreased in power. The base station can respondto the power up/power down indications by adjusting transmissions on theforward link by a set amount, e.g. ±1 db.

These four conditions can therefore be handled by encoding the heartbeatstandby channel 55HS or heartbeat request active channel 55HRA slotswith one of four different message codes. The message codes may beorthogonal codes, such as Walsh codes, or may be other types of codessuch as quasi-orthogonal PN codes, or PN codes.

The point is, both the desired heartbeat state, as well as the powercontrol feedback messages for the LQM channel can be handled by encodingthe transmissions in the appropriately assigned time slot 310 of theheartbeat standby channel 55HS or heartbeat request active channel55HRA.

This provides a method for providing closed loop feedback so that powercontrol on a commonly shared control channel, such as the LQM channel,may be implemented. Control is implemented on as fine a granularity as aper-user basis. Therefore, one major limitation of managing forward linkcapacity is eliminated, since only enough power needs to be allocated toeach forward link message to meet the minimum performance for individualusers in the network, rather than the minimum performance for all userscollectively in the network.

The principles of the present approach apply also to systems that usededicated control channels. For example, in IS-2000 Rev. C, afundamental channel is defined within an assigned traffic channel toprovide dedicated control. The encoded transmissions sent over the heartbeat channels of the present system can be used to provide closed looppower control of the dedicated control channels. That is, the inventiondoes not require the forward link to be a shared control channel.

In a preferred application, the heartbeat standby channel 55HS,heartbeat request active channel 55HRA and LQM channel 60L are alldefined by unique code such as long PN (Pseudo-Random Noise) codes.Accordingly, base station 25 detects a message from a field unit 42A inan assigned time slot by detecting whether or not a field unit 42Atransmits an RF (Radio Frequency) signal over the corresponding uniquelycoded channel. A transmission within an assigned time slot of eitherheartbeat channel need not include a meaningful data payload that mustbe demodulated because the mere coded RF transmission by a field unit42A within a time of a channel itself indicates to base station 25whether the corresponding field unit 42A desires to remain in thestandby mode or go active.

In one application, field unit 42A transmits unmodulated data includinga short PN code, a long PN code, and an orthogonal code such as a Walshcode in an assigned time slot 310 of the heartbeat-type channel, i.e.,the heartbeat standby channel 55HS of the heartbeat request activechannel 55HRA. Thus, the message as received in a time slot 310 iseasily identified without having to decode a corresponding data payloadmessage. A field unit 42A can then transmit at a lower power level thanwould otherwise be necessary if the field unit transmitted an indicationincluding a coded message or data payload.

Since field units 42 transmit during an assigned time segment over onlyone of the pair of heartbeat channels including heartbeat standbychannel 55HS and heartbeat request active channel 55HRA, the combinationof transmitted RF power on these channels is effectively that of asingle channel.

A marker is preferably included within a time slotted message of eitherheartbeat channel so that base station 25 can analyze whether acorresponding field unit 42 is properly synchronized. More specifically,field unit 42 transmits a marker at a predetermined position in a timeslot 310 and base station 25 then sends a message in the appropriatetime slot 310 of the forward link LQM channel 60L to indicate whetherthe field unit should advance or retreat its timing of future messagetransmissions.

Another aspect of the present invention involves maintaining a powerfeedback loop between each of multiple field units 42 and base station25. The indication transmitted in a time slot of the heartbeat standbychannel 55HS or heartbeat request active channel 55HRA is analyzed atbase station 25 to determine the strength of the received RF signal astransmitted by a corresponding field unit 42A. For example, the poweroutput of a field unit 42A can be adjusted based on a signal-to-noiseratio of the signal received at base station 25. If the signal strengthis lower than a desired level or outside a specified range as detectedby base station 25, a feedback message generated by base station 25 iscommunicated to the field unit 42A in the appropriate forward link foradjusting its power level for subsequent transmissions on the heartbeattype channels. In this way, the power level of a field unit 42A can beadjusted to reduce co-channel interference based on power adjustmentmessages transmitted to a field unit 42A in successive LQM time slots310. The power level of a transmitting device can be gradually increasedor decreased so that it has minimal impact on other channels.

The aforementioned method of adjusting the power output level of a fieldunit 42A is similar to the method as previously described forsynchronizing a field unit 42A to base station 25 via feedback messages.However, in the power feedback control loop, the power level output ofthe field unit 42A is adjusted via feedback messages instead of timing.Thus, the power level of a field unit 42A can be adjusted while in astandby mode so that, in the event that the transmitter goes activetransmitting a data payload to the receiver, the power level of thetransmitter is optimized to reduce co-channel interference.

The power feedback loop provides a reference for transmitting an RFsignal at a specified power level so that the field unit 42A candetermine at what level the field unit 42A should transmit an FEC(Forward Error Correction) coded message over other channels such asreverse link traffic channels. More specifically, a field unit 42Arecently assigned to the active mode can determine at what level totransmit a data payload to base station 25 depending on themodulation-type and FEC code to be used for transmitting the datapayload using the power level transmission on the heartbeat channel as areference.

Both power and timing feedback loops can be implemented simultaneouslyso that the power output level and timing of a field unit 42A isoptimized for potentially sporadic data transmissions. Thus, power andtiming of a field unit 42A is optimally adjusted even in a dynamicenvironment where the signal to noise ratio and signal path transmissiondelay of the field unit 42A changes almost instantaneously. A link ismaintained even during changing environmental conditions.

In the standby mode, power level optimization is achieved based uponmessages sent in the appropriate time slot 310 on the LQM channel 60Lindicating to a particular field unit 42A whether RF transmissions formthe field unit 42A to the base station 25 are received at an appropriatepower level. For example, signal transmissions from the field unittransmitter 40 to base station 25 are analyzed at base station receiver35 to achieve fine tuning power level adjustments for each of multiplefield units 42.

FIG. 3 is a timing diagram illustrating an exemplary mapping of bits ina forward link LQM time slot 310 according to the principles of thepresent invention. As shown, there are at least 16 bits transmitted ineach time slot 310, although this can vary depending on the application.

One bit of the LQM time slot 310 is LQM power level control bit 312 thatindicates whether a field unit transmission on the heartbeat standbychannel 55HS or heartbeat request active channel 55HRA received at basestation 25 in a previous Epoch cycle is detected to be within a desiredpower level range. This feedback message in the LQM time slot 310 ismonitored at the field unit 42A to adjust the power output level of thefield unit 42A so that power output of the field unit 42A is minimal butdetectable at base station 25. Of course, the power output level of thefield unit 42A is adjusted above the minimal detectable level so thattransmissions from the field unit 42A are still detectable even if thereis a slight change in environmental conditions.

Notably, if the transmission by a field unit 42A is so low that it isnot detectable at base station 25, a feedback message in the LQM timeslot 310 will be generated indicating that the field unit 42A shallincrease its power output level a predetermined amount so that the basestation 25 can hopefully detect a transmission by the field unit 42A ina following Epoch. Power feedback messages transmitted over multipleEpochs to the field unit 42A can be used to gradually adjust its poweroutput level. This gradual change in power output by the field unit 42Aminimally impacts the quality of other channels. In other words, thefield unit 42A preferably does not transmit at such a high power levelthat it causes undue interference with other field units 42 transmittingon other coded channels.

In a specific application, the LQM power level control bit 312 indicateswhether a field unit 42A is to increase or decrease its power leveloutput for transmissions on the reverse link 65. A logic one indicatesthat timing should be increased by, for example, ½ dB while a logic zeroindicates that power level output of the field unit 42A should bedecreased by ½ dB so that the received signal at base station 25 fallswithin a desired signal-to-noise ratio range. In this way, base station25 individually adjusts the power level of communication links betweenthe base station 25 and each of a plurality of field units 42. Saiddifferently, power output levels of corresponding field units 42 arefrequently adjusted, so that corresponding indications are received at adesired power level at base station 25. Consequently, the power outputlevel of a field unit 42 can be continuously adjusted so that it isoptimally set even though the field unit 42A may be moving very fastrelative to the base station 25, i.e., the reverse link path loss may bechanging and the power output level of the field unit 42A will beadjusted accordingly for supporting continued communications with basestation 25.

As mentioned, power adjustments are made at the field unit 42A basedupon the state of the LQM timing bit 312. Initially, timing is adjustedby a first predetermined amount such as ½ dB in the appropriatedirection depending on the state of this bit. However, if the field unit42A receives 8 “increase” power bits in a row or 8 “decrease” power bitsin a row over as many EPOCHs, power adjustments of the field unit 42Aare based on 1 db instead of ½ dB for the following LQM power controlbits 312 of the same state. In this way, the optimal power output levelof the field unit 42A can be achieved more quickly when the power levelfor a link is grossly out of adjustment.

Once the field unit 42A determines that the power output level isovercorrected, i.e., the polarity of the LQM timing bit 312 changesstate from one epoch to the next, power output adjustments at the fieldunit 42A revert back to ½ db for each subsequently received LQM powercontrol bit 312. When power synchronization is achieved between a fieldunit 42 and base station 25, the LQM power control bit 312 willtypically be set to alternating logic ones and zeros for severalsuccessive EPOCH cycles. In other words, power control output at thefield unit will jitter ½ dB when synchronization is practically achievedbetween the base station 25 and field unit 42A. This amount of jitter istolerable for maintaining such synchronization links. Of course, afilter can be implemented at the field unit 42A so that the power outputdoes not jitter from one Epoch to the next of the filed unit 42A.

Rather than transmit a single LQM power control bit 312, the LQM timingslot 310 can also include a multi-bit message indicating an amount thatthe corresponding field unit 42A is to increase or decrease it poweroutput level.

FIG. 4 is a timing diagram illustrating a field unit 42A requesting tobe assigned reverse link traffic channels according to the principles ofthe present invention. As shown, a field unit 42A in the standby mode isassigned a particular time slot 310 in Epoch E₁. As previouslydiscussed, the field unit 42A transmits over the assigned time slot 310of the heartbeat standby channel 55HS to remain in the standby mode. Inresponse to this reverse link indication from the field unit 42A, basestation 25 transmits a feedback message in the appropriate time slot 310of the LQM channel 60L in Epoch E₁ for maintaining synchronization ofthe link. As discussed, this feedback message can include both power andtiming control adjustment information.

Epoch E₂ illustrates a similar circumstance where the field unit 42Acontinues requesting to remain in the standby mode. Consequently, therepetitive function of monitoring a timing marker within a time slot 310and providing corresponding feedback in the reverse link over the LQMchannel 60L ensures that the corresponding link is synchronized in theevent that the field unit 42 desires to transmit a data payload in areverse link direction.

In following Epoch E₃, field unit 42A indicates to base station 25 arequest to go active so that it will be assigned reverse link trafficchannels to transmit a data payload. As mentioned, this is achieved bygenerating an RF signal in the appropriate time slot 310 of theheartbeat request active channel 55HRA. Depending on a number ofavailable reverse link traffic channels, there can be a delay betweenthe time a field unit requests to go active and the time trafficchannels are actually assigned for use by the field unit 42A. Thus, itis desirable to repeat a request to go active by transmitting in anassigned time slot 310 of the heartbeat request active channel 55HRA atthe base station 25. Since timing adjustment feedback messages are alsotransmitted to the field unit 42A based on messages received on theheartbeat request active channel 55HRA, precise synchronization andpower control of the corresponding link between base station 25 andfield unit 42 is maintained for subsequent Epochs E₄ and E₅.

Prior to or during Epoch E₅, field unit 42A is notified which trafficchannels are allocated for transmitting its data payload in a reverselink direction.

Epoch E₆ and E₇ illustrate that field unit 42A has been assigned use ofreverse link traffic channels 55T for transmitting a data payload.Notably, the field unit 42A no longer transmits an indication to thebase station 25 over either the heartbeat standby channel 55HS or theheartbeat request active channel 55HRA. However, a link quality messageis still transmitted in a forward link direction from the base station25 to adjust timing of the field unit 42. The timing adjustment feedbackmessages are based on markers transmitted over the reverse link trafficchannels 55T. As shown, in Epoch E₆ and E₇, an LQM message istransmitted to the field unit 42A in a newly assigned active time slotbetween A₁ and A₁₆. Thus, transmissions from the base station 25 tofield unit 42A have shifted to a new time slot 310. Of course, prior toEpoch E₆, field unit 42A must be notified of the traffic channels 55T onwhich it is to transmit a data payload and the newly assigned activetime slot 310 in which the field unit 42A is to receive a time-slottedLQM message.

As mentioned, markers are included with the data payload transmissionsover the reverse link traffic channel 55T to base station 25 where theyare analyzed. In this instance, the minimal feedback timing adjustmentmessages are generated based on the markers received within the reverselink traffic channels 55T. The timing adjustment messages aretransmitted in the newly assigned active time slot A₁ of the forwardlink LQM channel 60L.

After a data payload is transmitted over the reverse link trafficchannel 55T, the field unit 42A is placed in the standby mode as shownin Epoch E₈ and E₉. Accordingly, synchronization is again maintainedbased on a feedback loop between a field unit 42A and base station 25.More specifically, messages transmitted in a time slot 310 of theheartbeat standby channel 55HS are again analyzed at base station 25 andtiming adjustment feedback information is transmitted in the standbytime slot S₁ of the forward link LQM channel 60L to preciselysynchronize the field unit 42A with the base station 25.

FIG. 5 more particularly shows hardware components at base station 25that are used to achieve synchronization and power control of thereverse link 65 and forward link 70. Information transmitted in a timeslot 310 as assigned for use by a field unit 42A is analyzed by acorresponding heartbeat correlation filter such as heartbeat standbycorrelation filter 440 or heartbeat request active correlation filter445. Generally, the unique codes of each heartbeat channel are monitoredin different time slots 310 to detect a request by a corresponding fieldunit 42A to be placed in the active mode or remain in the standby mode.Thereafter, the base station 25 will set field unit 42A to the activemode by assigning it the appropriate resources if a request to go activeis detected. Note that heartbeat standby correlation filter 440 is usedto identify a long PN code corresponding to a request by the field unit42A to remain in the standby mode, while heartbeat request activecorrelation filter 445 at base station 25 identifies a long PN codecorresponding with a request to be placed in the active mode.

Regardless on which heartbeat-type channel a field unit 42A transmits inan assigned time slot 310, the marker from the field unit 42A ismonitored by a pulse timing analyzer 422. It is then determined whetherthe message transmission by a corresponding field unit 42A is receivedearly or late within a time slot 310 at base station 25. Preferably, thestrongest received diversity string in a time slot 310 will bedesignated as the time alignment string for analyzing timing of themessage received over heartbeat standby channel 55HS or heartbeatrequest active channel 55HRA.

Time slot alignment is preferably based on the correlation profile ofthe pilot in a particular string, which is analyzed using correlationfilters as mentioned. The output of the correlation filters 440, 445include 256 samples, which represent 64 lags at 4 samples per lag. The256 sample output-window represents the total correlation time span ofthe base station 25. Preferably, the time alignment point or marker in atime slot 310 is sample number 80, which allows 20 lags for precursorand 44 lags for post cursor channel information.

Generally, the computation of the time alignment error is based on adetermination of where the centroid or peak lies in a given samplestring. For example, each field unit 42A transmitting in its assignedtime slot 310 over either the heartbeat standby channel 55HS orheartbeat request active channel 55HRA includes a marker, i.e., the peaksignal, located at a predetermined position within a time slot. Thestrongest pilot path for the channel and 2 samples on either side of themain path, i.e., 1 and ¼ chips, is statistically analyzed to determinethe centroid or peak of a marker within a time slot. The centroid of thesamples in FIG. 7 are calculated based on the following equation:$L = \frac{\sum\left\lbrack {t \times {Q(t)}} \right\rbrack}{\sum{Q(t)}}$where L is a position of the centroid in a time slot, t is the sampletime along the X-axis, and Q(t) is the magnitude of a sample at a givensample time. For example, L is calculated based on the results as shownin FIG. 7:$L = \frac{\left( {{.25}*76} \right) + \left( {5*77} \right) + \left( {1.0*78} \right) + \left( {{.8}*79} \right) + \left( {{.6}*80} \right)}{{.25} + {.5} + 1.0 + {.8} + {.6}}$L = 78.317

Again, the timing alignment error is determined by comparing the timingof the computed centroid to the desired time set point of 80, which ischosen as the reference point for timing alignment within a time slot310. Since the centroid in the example above is estimated to be 78.317,timing is early and, therefore, the corresponding LQM timing bit 311will be set to a logic “one” indicating that the corresponding fieldunit should advance its timing reference by ⅛ of a chip so thatsubsequent messages are transmitted ⅛ of a chip later in time slot 310.This overall feedback technique in the present invention ensurescontinuous fine-tuning the time alignment between base unit 25 and eachof multiple field units 42.

Preferably, the time error is calculated by taking the integer of twicethe difference between the desired set point sample 80 and L. Forexample,time_error=integer [(L−80)*2]

If the time_error result is negative, the LQM timing bit 311 is set to alogic “one.” Conversely, the LQM timing bit 311 is set to a logic “zero”when time_error is positive.

Referring again to FIG. 5, processor 426 analyzes timing data andgenerates time_error for synchronizing the reverse link heartbeatchannels 55H and forward link LQM channel 60L. LQM time slotted messagesare then transmitted by LQM signal generator 450 on LQM channel #1 60Lto provide timing adjustments for the corresponding field unit 42A asmentioned.

If a field unit 42A in the standby mode transmits a request to go activeby transmitting in an assigned time slot of the heartbeat request activechannel 55HRA, such a request is detected at heartbeat request activecorrelation filter 445. As previously discussed, the timingcharacteristics of an active mode request detected at heartbeat requestactive correlation filter 445 is also analyzed to determine timingerrors as described above for maintaining alignment on a particular linkbetween the base station 25 and each field unit 42A.

If resources are available for allocating traffic channels 55T, therequesting field unit 42A is placed in the active mode by base station25, where configuration details for setting up the data transfer arehandled by processor 426. For example, information regarding new LQMtime slot assignments, i.e., assignment of an active mode time slot A₁ .. . A₁₆, is sent to a corresponding field unit 42A over, for example,the paging channel 60P. Reverse link traffic channels 55T are thenallocated for transferring a data payload from field unit 42A to basestation 25.

While in the active mode, synchronization of the forward and reverselink is maintained based on messages transmitted over the LQM channel60L and traffic channels 55T since the heartbeat channel time slot is nolonger dedicated on the reverse link 65 for use by the transmittingfield unit 42A. More specifically, a timing marker is included in thereverse link traffic channel transmissions so that base station 25 canmonitor whether data payload field unit 42A is early or late in itstiming.

Messages transmitted by a field unit 42A in the active mode aretransmitted to base station 25 over traffic channels 55T and thecorresponding traffic channel signal is fed into the traffic channelcorrelation filter 430 at base station 25 for detection of pilot symboltiming markers. Preferably, a field unit 42A transmits a sequence of 32pilot symbols in an assigned time slot 310 as a timing marker. Thetraffic channel 55T is then analyzed by pulse timing analyzer 420 todetermine whether such messages are early or late with respect to adesired synchronization of the field unit 42 with base station 25.

The process of analyzing a pulse or marker for estimating the centroidis similar to that described earlier in the specification for messagesand corresponding markers such as long PN codes on either heartbeatchannel 55HS or 55HRA. However, when field unit 42A is in the activemode, pilot symbols in the traffic channels 55T are used as a timingreference mark rather than long PN codes. Again, see FIG. 7 and relateddiscussion above for details regarding how a timing marker is analyzedto identify whether a field unit 42A should advance or retard itstiming.

FIG. 8 is a table illustrating different operational modes according tothe principles of the present invention and how synchronization ismaintained between a field unit and base station for each of the modes.

Preferably, timing alignment of the base station 25 and field units 42is based upon the LQM timing bit 311 as transmitted in an assignedactive time slot A₁ . . . A₁₆ on the forward link 70. When the receiptof data messages transmitted by the active field unit 42 are receivedearly or late with respect to an assigned time slot, the LQM timing bit311 is set accordingly to advance or retard timing of future messagetransmissions on the traffic channels 55T.

Although a single traffic channel correlation filter 430 is shown fordetecting a marker in a single traffic channel 55T, multiple trafficchannels 55T are optionally analyzed to coordinate timing alignmentbetween the reverse link 65 and forward link 70.

As mentioned, access channel 55A is used by the field units 42 totransmit requests for establishing a synchronization link with the basestation 25. Typically, messages on the access channel 55A aretransmitted on a random basis. Hence, a message collision may occur iftwo or more link requesting field units 42 happen to transmit a linkrequest message on the access channel 55A at the same time.

If a collision is detected on the access channel 55A, the collision ismade known to the field units 42 based upon a message generated bypaging channel signal generator 455 over paging channel 60P. Each fieldunit 42 will then retransmit their request to establish asynchronization link on the access channel 55A based on a random backoff time, making it less likely that a collision will occur on a secondor other subsequent attempt.

Access channel 55A, also shown in FIG. 5, is fed into access channelcorrelation filter 435. Preferably, a field unit 42 transmits a sequenceof 32 pilot symbols including information identifying the field unit 42Arequesting a synchronization link. A received sequence of pilot symbolsis analyzed by pulse timing analyzer 422 to determine initial timinginformation of the field unit 42A with respect to the base station 25.Since the field units 42 randomly transmit requests on the accesschannel 55A, it is necessary to determine an initial timing errorbetween the field unit 42 and base station 25 for achieving a coarsesynchronization of the forward and reverse link channels.

If it is determined by the base station 25 that a synchronization linkwill be established between the base station 25 and requesting fieldunit 42A, an appropriate acknowledgment message is transmitted over theforward paging channel 60P to the base station 25 to the correspondingfield unit 42A. Among other information transmitted over the forwardpaging channel 60P to the field unit 42, a heartbeat time slotassignment, an LQM time slot assignment, and synchronization informationsuch as coarse timing adjustment information is also transmitted to thefield unit 42. Thus, a field unit 42A newly assigned to the standby modecan transmit an indication over one of the heartbeat-type channels formaintaining more precise synchronization with base station 25.

As mentioned, coarse timing adjustment information is transmitted on theforward paging channel 60P to roughly synchronize the link requestingfield unit 42A with respect to base station 25. Preferably, a 10-bitsigned number is transmitted to the field unit 42A indicating an amountto advance or retard its timing with respect to the link request messageof the field unit 42 as previously transmitted on the access channel55A. Each least significant bit (LSB) in the 10-bit signed number isappropriately weighted. For example, an LSB can represent 16 chips.Based on this timing correction information, the corresponding fieldunit 42A adjusts its coarse timing relative to the base station 25.Thereafter, messages are then transmitted in the appropriate reverselink time slot of the heartbeat channel 55HS, 55HRA or traffic channel55T. Fine-tuning is thereafter achieved by analyzing transmissions byfield unit 42A at base station 25 and providing synchronizationinformation over the LQM channel 60L feedback path.

In addition to transmitting in the appropriate time slot, coarse andfine synchronization with the base station 25 renders it possible for afield unit 42 to receive information in its assigned time slot in theforward link.

FIGS. 6A and 6B are a flow charts providing details of how a wirelesscommunication link is established between field unit 42A and basestation 25. There are typically multiple field units 42 requestingcommunication links in a particular service area, where each mobile orfield unit 42A is located at a different distance with respect to basestation 25. For example, some field units 42 can be located very closeto base station 25 while others are located very far away. Hence, thetime it takes for a signal to travel from a particular field unit 42A tobase station 25 is different for each field unit 42. Precise timingalignment of a specific field unit 42 and the base station 25 istherefore important to avoid or minimize collisions between field units42 transmitting in adjacent time slots.

If all field units 42 transmitted in real time without taking intoaccount the distance to base station 25 and corresponding delay, messagetransmissions in an assigned time slot from a particular field unitwould be skewed, i.e., messages at the base station would be receivedslightly out of an assigned time slot. Therefore, message transmissionsfrom each field unit 42A are precisely adjusted as previously discussedto prevent this skewing phenomenon.

Not only does distance from a field unit 42A to base station 25 effecttiming alignment, so does the environment in which a field unit 42Atransmits a message. For example, building structures, atmosphericconditions and other geographical terrain will effect the path of asignal transmitted from a field unit 42A to base station 25. Therefore afield unit 42 changing position merely a few feet in several seconds canhave a substantial impact on timing of a signal path, thus, effectingtiming alignment between a reverse link 65 and forward link 70. Based onthe principles of the present invention, the previously described methodof continuously adjusting timing transmissions in the shared reversechannel 65 minimizes collisions among multiple field units 42transmitting to base station 25 in adjacent time slots.

Step 510 in FIG. 6A shows an entry point of the flow chart forestablishing a wireless communication link. In step 515, access channel55A is monitored by base station 25 to detect requests by field units 42to establish wireless synchronization links with base station 25. A linkrequest message received at base station 25 includes a sequence of pilotsymbols followed by data identifying the link requesting field unit 42A.Based on the data information received over access channel 55A, basestation 25 is able to access characteristics of the corresponding fieldunit 42.

If no standby time slots are available for establishing a newsynchronization link, the connection request by a field unit 42A isdenied as shown in step 525. A message is then transmitted to thecorresponding field unit 42A on the forward link paging channel 60P toindicate that no time slots are available and the field unit 42A musttry again at a later time to establish a standby synchronization link.

If resources are available to establish a new link in step 520, basestation 25 analyzes the timing of the request message as received from afield unit 42A on access channel 55A in step 530. As mentioned, thesequence of 32 pilot symbols are analyzed to determine the location ofthe peak pulse or marker in the reverse link 65. Based on the time whenthis random message is received with respect to the base station'smaster time reference EPOCH mark, M1, and the distance that the fieldunit 42A is located from base station 25, a coarse time adjustmentmessage is generated by the base station 25 to synchronize timingbetween the link requesting field unit 42A and base station 25. Thiscoarse timing information, preferably a 10-bit signed number indicatinghow a field unit 42 should adjust its timing to align the field unitwith the base station EPOCH mark, is sent to the field unit 42A over theforward link paging channel 60P in step 535. The field unit 42A thenadjusts its timing reference accordingly so that subsequent messages aretransmitted in an assigned time slot on the reverse link 65. Timingalignment also ensures that the field unit 42A can receive messages frombase station 25 in the appropriate time slot of the forward link LQMchannel 60L.

Following in step 540, base station 25 assigns two time slots to thelink requesting field unit 42A over paging channel 60P. One time-slotassignment indicates the time slot in which the field unit 42A is toreceive LQM messages from the base station 25 over the LQM channel 60L.Another time-slot assignment indicates in which time slot 310 of thereverse link field unit 42 is to transmit over a heartbeat-type channelto base station 25. Based upon these time slot assignments, the basestation 25 and field units 42 can determine to which link a messagepertains as the time slot itself indicates to which target a message isdirected.

While in the standby mode, base station 25 monitors periodic messages inan assigned time slot for a transmission on either the heartbeat standbychannel 55HS or heartbeat request active channel 55HRA by acorresponding field unit 42A. For example, a marker received in a timeslot of either channel is analyzed at base station 25 to correct timingalignment as mentioned between base station 25 and field unit 42A. Ifthe message in a time slot is received early or late at base station 25,timing of future transmissions by the field unit 42 in an assigned timeslot 310 on a reverse link heartbeat channel is appropriately retardedor advanced based upon the LQM timing bit 311 for a particular fieldunit 42A in step 542.

Timing adjustments are made at the field unit 42A based upon the stateof the LQM timing bit 311. Initially, timing is adjusted by ⅛ of a chipin the appropriate direction depending on the state of this bit.However, if the field unit 42A receives 8 retard bits in a row or 8advance bits in a row over as many EPOCHs, timing adjustments of thereference at the field unit 42A at are based on ½ of a chip instead of ⅛of a chip for the following LQM bits 311 of the same state. In this way,synchronization between the base station 25 and field unit 42 isachieved more quickly when timing for a link is grossly out ofadjustment.

Once the field unit 42A determines that timing is overcorrected, i.e.,the polarity of the LQM timing bit 311 changes state from one epoch tothe next, timing adjustments at the field unit 42 revert back to ⅛ of achip for each subsequently received LQM timing bit 311. Whensynchronization is achieved between a field unit 42 and base station 25,the LQM timing bit 311 will typically be set to alternating logic onesand zeros for several successive EPOCH cycles. In other words, timing atthe field unit will jitter ⅛ of a chip when synchronization ispractically achieved between the base station 25 and field unit 42A.This amount of jitter is tolerable for maintaining such synchronizationlinks.

If field unit 42A receives another 8 cycles of timing adjustmentcorrections in the same direction such that 16 successive LQM bits 311are the same state, the time adjust correction is set to 1 chip perreceived LQM timing bit 311. As stated earlier, when over-correction isdetected, timing adjustments at the field unit are again based on ⅛ of achip for each received LQM timing bit 311 again.

In addition to monitoring timing pulses for aligning messagetransmissions of each field unit 42, base station 25 also determines onwhich heartbeat channel a field unit 42A transmits during its assignedtime segment T_(SLOT). It is then determined in step 547 whether a fieldunit 42A requests to be set to the active mode based on whether thefield unit 42A transmits over the heartbeat request active channel55HRA. If so, the base station allocates appropriate resources such astraffic channels 55T in the reverse link 65 to support the data transferin step 550. Additionally, base station 25 is assigned an active timeslot for use by a field unit 42, i.e., one available time slot betweenA₁-A₁₆, in the forward link LQM channel 60L to maintain asynchronization loop. While in the active mode, as mentioned, the fieldunit 42A maintains synchronization with base station 25 based on asequence of well-placed pilot symbol markers in the traffic channels55T, upon which the base station 25 issues timing adjustments in theappropriate time slot 310 using the forward link LQM timing bit 311.Additionally, the field unit 42A transmits data over the reverse linktraffic channels 55T in step 555 before returning to the main loop againat step 560. At this re-entry point into the main loop again, the fieldunit 42 is then reassigned a standby mode time slot 310.

If a field unit 42A has not been in the standby mode too long in step560, base station 25 determines whether the field unit 42A has made arequest to terminate a wireless link between base station 25 andcorresponding field unit 42A in step 572. Without the request to teardown a link, processing loops back to step 542.

If field unit 42 generates a request to tear down a corresponding linkin step 572, base station 25 acknowledges such a request in step 575 bysending a message to the field unit 42 and tears down the communicationlink. This is one way of terminating the flow chart as shown in step580.

Referring again to step 560, if it is determined that the field unit 42Ais inactive too long, i.e., in standby mode not transmitting data, thebase station revokes the assigned LQM and heartbeat channel slots foruse by other users and maintains an idle connection with the field unit42A in step 565.

When it is determined that field unit 42A requests to go active again instep 582, process flow continues at the beginning of the flow chart toreestablish a synchronized link in step 570. In such a case,connectivity is reestablished based in part on the prior connection. Forexample, it is not necessary to go through the entire configurationprocess since data maintained with respect to the corresponding recentlyactive link is advantageously used to minimize the overhead associatedwith reviving the previous connection.

Flow continues at step 585 if base station 25 fails to detect a requestby the field unit 42 to go active again in step 582. If base station 25fails to receive a response from an idle field unit 42A in a specifiedtime out period in step 585, base station 25 pings the field unit 42 onforward page channel 60P to elicit a response by the field unit 42 instep 587. If the field unit 42A does not respond in step 590, it isassumed that the field unit 42A is shut down and an idle connection istherefore no longer maintained for that particular field unit 42. If thefield unit 42A responds to the ping in step 590, process flow continuesin step 595 at START (step 510) of the flowchart to reestablish the linkas a standby connection.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. A method for supporting wireless communications, the methodcomprising: assigning a field unit a first time slot in a reverse linkchannel; receiving and analyzing messages from said field unit at a basestation in the first time slot; and maintaining synchronization betweensaid field unit and said base station when said field unit is inactiveor standby mode.
 2. The method of claim 1 further comprising: assigningsaid field unit a second time slot in a forward link channel.
 3. Themethod of 2, wherein said field unit transmits a timing reference signalin said first time slot when said first unit is in standby mode.
 4. Themethod of claim 2 further comprising: said field unit indicating to saidbase station to remain in the standby mode.
 5. The method of claim 2,wherein said reverse link channel is a heartbeat-type channel.
 6. Themethod of claim 5, wherein said heartbeat-type channel comprises anactive channel and a standby channel.
 7. The method of claim 6 furthercomprising: transmitting in said standby channel and indication toremain in the standby mode.
 8. The method of claim 6 further comprising:transmitting in said active channel a request for allocation of reverselink traffic channels for transmitting data.
 9. The method of claim 6further comprising: transmitting power control information includingwhether messages over said forward link channel should be increased inpower or decreased in power.
 10. The method of claim 9 wherein saidstandby channel and said active channel include said power controlinformation.
 11. A system for supporting wireless communicationcomprising: a base station for assigning a field unit a first time slotin a reverse link channel, and receiving and analyzing messages fromsaid field unit in said first time slot; and said base station and saidfield unit maintaining synchronization when said field unit is in anactive mode or standby mode.
 12. The system of claim 11 furthercomprising said base station for: assigning said field unit a secondtime slot in a forward link channel.
 13. The system of claim 12, whereinsaid field unit transmits a timing reference signal in said first timeslot when said field unit is in standby mode.
 14. The system of claim 12further comprising: said field unit indicating to said base station toremain in the standby mode.
 15. The system of claim 12, wherein saidreverse link channel is a heartbeat-type channel.
 16. The system ofclaim 15, wherein said heartbeat-type channel comprises an activechannel and a standby channel.
 17. The system of claim 16 furthercomprising: transmitting in said standby channel and indication toremain in the standby mode.
 18. The system of claim 16 furthercomprising: transmitting in said active channel a request for allocationof reverse link traffic channels for transmitting data.
 19. The systemof claim 16 further comprising: transmitting power control informationincluding whether messages over said forward link channel should beincreased in power or decreased in power.
 20. The system of claim 19wherein said standby channel and said active channel include said powercontrol information.